U.S. patent number 5,091,075 [Application Number 07/549,324] was granted by the patent office on 1992-02-25 for reforming process with improved vertical heat exchangers.
This patent grant is currently assigned to UOP. Invention is credited to Thomas J. Godry, Patrick S. O'Neill, Elias G. Ragi.
United States Patent |
5,091,075 |
O'Neill , et al. |
February 25, 1992 |
Reforming process with improved vertical heat exchangers
Abstract
A process for the reforming of hydrocarbons is improved by the
use of an enhanced nucleate boiling surface in a selected portion
of the feed effluent heat exchanger. In a vertical type heat
exchanger where the reforming feedstream enters at a lower end of
the heat exchanger and is at least partially vaporized in the heat
exchanger by contact with a reforming effluent stream that enters
an upper end of the heat exchanger and is at least partially
condensed therein, an enhanced nucleate boiling surface is formed
on the heat exchange surface that is in contact with the entering
liquid phase portion of the stream feed. The enhanced nucleate
boiling surface increases the amount of condensing that takes place
on the opposite side of the heat exchange surface in a
boiling-condensing zone. The use of the enhanced nucleate boiling
surface in the boiling zone of the heat exchanger not only improves
the heat transfer coefficient on the boiling side of the tube wall
surface, but also the overall heat transfer on the opposite
condensing side of the tube wall surface. The addition of the
enhanced nucleate boiling surface provides a substantial increase
in the overall heat exchange and the overall heat transfer
coefficients for the heat exchanger.
Inventors: |
O'Neill; Patrick S.
(Williamsville, NY), Ragi; Elias G. (Williamsville, NY),
Godry; Thomas J. (Tonawanda, NY) |
Assignee: |
UOP (Des Plaines, IL)
|
Family
ID: |
24192522 |
Appl.
No.: |
07/549,324 |
Filed: |
July 6, 1990 |
Current U.S.
Class: |
208/134; 165/133;
208/135; 208/136; 208/137; 208/138; 208/139; 208/140; 208/141;
208/142 |
Current CPC
Class: |
C10G
35/04 (20130101) |
Current International
Class: |
C10G
35/04 (20060101); C10G 35/00 (20060101); C10G
035/06 () |
Field of
Search: |
;208/134,135,136,137,138,139,140,141,142 ;165/133 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Myers; Helane E.
Attorney, Agent or Firm: McBride; Thomas K. Tolomei; John
G.
Claims
We claim:
1. In a process for reforming hydrocarbons comprising contacting a
reforming feedstream with a reforming catalyst in a reforming
reaction zone to form a reforming effluent stream wherein heat from
the reforming effluent stream is transferred to the reforming
feedstream by indirect heat exchange in a vertical heat exchange
zone, the reforming feedstream is passed to a lower end of the
vertical heat exchange zone and at least partially vaporized in
said heat exchange zone, the reforming effluent stream is passed to
an upper end of the vertical heat exchange zone and at least
partially condensed in said heat exchange zone and boiling of said
feedstream and condensing of said effluent stream occurs in a lower
portion of said heat exchange zone, the improvement comprising
contacting said reforming feedstream with an enhanced nucleate
boiling surface in a lower portion of said heat exchange zone and
maintaining said contact of the feedstream with said nucleate
boiling surface above the point in said vertical heat exchange zone
where boiling of said feedstream occurs to increase the
effectiveness of the condensing heat transfer in said heat
exchanger.
2. The process of claim 1 wherein said enhanced nucleate boiling
surface is a High Flux boiling surface and said feedstream contacts
a layer of reentrant openings in said lower portion of said
vertical heat exchange zone.
3. The process of claim 1 wherein boiling, condensing and gas-gas
heat transfer occurs in said vertical heat exchange zone.
4. The process of claim 3 wherein said feedstream contacts said
nucleate boiling surface for at least half the length of said
vertical heat exchange zone.
5. The process of claim 1 wherein said reforming feed has a higher
dewpoint than said reforming effluent.
6. The process of claim 5 wherein said reforming feed is vaporized
and reforming effluent is condensed in a boiling-condensing section
of said vertical heat exchange zone and said boiling-condensing
section includes an upper portion that does not contain saturated
reforming effluent vapors.
7. The process of claim 6 wherein said upper portion extends for an
average length of at least 6 inches.
8. In a process for reforming hydrocarbons comprising combining a
liquid phase reforming feedstream and hydrogen to form a combined
reforming feed having a first dewpoint, passing the combined
reforming feed into the bottom of a vertically oriented, heat
exchange zone to heat and vaporize the feed, passing the vaporized
feed to a reforming reaction zone, and contacting said vaporized
feed with a reforming catalyst in said reforming reaction zone to
form a gas phase reforming effluent stream having a second dewpoint
that is lower than said first dewpoint, passing said gas phase
effluent stream to the top of said heat exchange zone, and
transferring heat from the reforming effluent stream to the
reforming feedstream by indirect heat exchange in said heat
exchange zone to at least partially condense said effluent stream
in a lower portion of the heat exchange zone, the improvement
comprising contacting said reforming feedstream with an enhanced
nucleate boiling surface formed on a concave portion of a heat
exchange surface in said heat exchange zone and extending upward
from the bottom of said heat exchange zone past the point in said
heat exchange zone where boiling of said feedstream occurs to
increase the effectiveness of the boiling and condensing heat
transfer in the lower portion of the heat exchange zone.
9. The process of claim 8 wherein said enhanced nucleate boiling
surface is a High Flux boiling surface and said feedstream contacts
a layer of reentrant openings in said vertical heat exchange
zone.
10. The process of claim 8 wherein at least 30 wt. % of said
reforming effluent stream is condensed in said heat exchange
zone.
11. The process of claim 8 wherein the reforming feedstream boils
and the reforming effluent stream is condensed in a
boiling-condensing section of said heat exchange zone and said
section has a lower portion containing saturated reforming effluent
vapor and an upper portion that is relatively free of saturated
reforming effluent vapor.
12. The process of claim 11 wherein said upper portion extends for
a length equal to at least 10% of the total length of the
boiling-condensing section.
13. The process of claim 11 wherein said upper portion extends for
a length of at least 6 inches.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of reforming hydrocarbons in
the presence of a reforming catalyst. More specifically, this
invention relates to heat exchange between a reforming feedstream
and a reforming effluent stream in a reforming process.
2. Discussion of the Prior Art
Catalytic reforming is a well-known process used in petroleum
refineries to increase the octane number of straight run
distillates (naphthas) by promoting chemical reactions which reduce
the paraffin content and increase the content of aromatics and
isoparaffin fractions. The desired chemical reactions are carried
out over a catalyst at temperatures in excess of 900.degree. F.,
and in the most common processes, at pressures less than 150 psi.
In a typical reformer a hydrotreated naphtha with a 380.degree. F.
end point is mixed with a recycle gas which is rich in hydrogen and
heated by indirect heat exchange with gaseous products from the
reactor in a feed-effluent exchanger. About 30-40% of the heat
transferred goes to vaporize the liquid feed. After the combined
feed leaves the feed-effluent exchangers it enters a preheater that
raises the feed to the desired reactor temperature.
The major reactions that occur in the reformer are the
dehydrogenation of naphthenes to form aromatics, the
dehydrocyclization of paraffins to form aromatics, the
isomerization of paraffins, and the hydrocracking of higher boiling
fractions to form paraffins in butane. The dehydrogenation and
dehydrocyclization reactions are endothermic and evolve hydrogen.
These reactions are favored by reduced pressure, reduced space
velocity and high temperature. The paraffin isomerization reaction
is slightly endothermic and is not significantly influenced by
pressure in the reforming zone. The hydrocracking reactions are
undesirable exothermic reactions that are favored by increased
pressure and low space velocity. Due to the nature of the above
reactions, it is desirable to operate the process at as a low a
pressure as possible. Lower pressures, however, increase the amount
of coke deposited on the catalyst thereby reducing its
effectiveness. Coking problems can be reduced by increasing the
ratio of hydrogen to hydrocarbons in the feed or by using more
coke-tolerant catalyst. Further information on reforming processes
may be found in U.S. Pat. Nos. 4,119,526; 4,409,095 and
4,440,626.
Proper heat recovery in the feed effluent exchanger is important to
the efficiency of product recovery in the reforming process. After
the reaction occurs, the hot effluent exchanges heat with the feed
by first cooling to the dewpoint and then partially condensing. The
higher the feed outlet temperature to the preheater, the less fuel
must be burned to maintain reactor temperature. Also, the more
condensation that occurs in the effluent stream in the exchanger,
the less downstream cooling must be provided to separate the
reformate from the recycle gas.
It has been difficult by conventional methods to increase the
outlet temperature of the reforming feedstream or the amount of
condensation that occurs in the effluent stream. The typical
approach of a person skilled in the design of feed effluent
changers, when attempting to decrease the temperature difference
between the feed leaving and the effluent entering the heat
exchanger, is to increase the heat recovery or capacity of the
existing system by adding surface area, either in the form of
longer exchangers or additional units in parallel. Both approaches
are costly and have technical disadvantages. Firstly, increasing
the exchanger length would result in additional pressure drop that
translates to increased recycle gas compressor power and reduced
delivery pressure to downstream processes. Secondly, the addition
of extra tubes, either as larger diameter exchangers or more units
in parallel, would result in lower fluid velocities since the total
stream is split into more paths. The resulting lower velocities
would in turn lead to lower heat transfer coefficients which would
work against the addition of surface area and also increase the
likelihood of undesirable maldistribution of the 2-phase feedstream
at the inlet. Finally, the designer would face the problem of
reduced available temperature difference between the feed and
effluent streams. Each increment in heat recovery gained by the
addition of ordinary surface area results in a reduction of the
temperature difference. Accordingly, the designer is confronted
with an approaching temperature pinch and declining heat transfer
coefficients with lower velocities and would likely conclude that
exorbitant amounts of additional bare tube surface area must be
added to gain a very modest increase in heat recovery and,
therefore, is not practical or feasible.
It is known in the art that the surface of heat exchanger tubes can
be treated to promote nucleate boiling. Such nucleate boiling
surfaces can promote dramatic increases in the boiling film
coefficients that are associated with heat transfer tubes in a
boiling heat exchange zone. Such enhanced boiling surface for heat
exchange tubes are discussed in U.S. Pat. Nos. 3,384,154;
3,821,018; 4,064,914; 4,060,125; 3,906,604; 4,216,826 and
3,454,081. Such surfaces have been known to provide benefits to a
variety of processes. For example, a significant improvement in the
refrigeration of an alkylation effluent by the use of an enhanced
boiling surface is taught in U.S. Pat. No. 4,769,511. Such nucleate
boiling surfaces have been known to increase boiling film
coefficients by a factor of 10 or more.
It is an object of this invention to increase the heat transfer
between a reforming feedstream and a reforming effluent stream.
It is a further object of this invention to reduce the temperature
differential between an existing reforming feedstream that is
exchanged with an entering reforming effluent stream.
A further object of this invention is to increase the feed
processing capacity of a reforming process while maintaining the
same feed outlet temperature and effluent inlet temperature and the
same surface area in a feed effluent heat exchanger.
BRIEF SUMMARY OF THE INVENTION
In this invention a reforming process is improved by the use of an
enhanced nucleate boiling surface in a feed effluent exchanger of
the reforming process. The nucleate boiling surface is used in a
boiling-condensing zone of the heat exchanger where an enhanced
section of the heat exchange surface vaporizes a liquid phase
component of the feedstream and the opposite of the heat exchange
surface is condensing the effluent stream. The improvement to the
process uses an enhanced nucleate boiling surface to increase the
boiling film coefficient in that section of the heat exchange zone
where boiling of the liquid phase feed components occurs. The
improved heat recovery in the boiling zone of the heat exchanger
has the unexpected result of causing the effluent stream that is
cooling and partially condensing on the opposite side of the heat
exchange surface to undergo additional condensation. This
additional condensation takes place under conditions that promote
high heat transfer coefficients, thereby resulting in a significant
additional overall benefit to the performance of the heat
exchanger.
The effluent begins to condense not when its bulk vapor temperature
reaches the dewpoint but when the tube metal temperature is at the
dewpoint. Since heat transfer coefficients for partial condensing
in the presence of a high thermal conductivity gas, such as
hydrogen, are substantially higher than those of pure cooling, the
net result is additional improvement in the zone overall heat
transfer coefficient beyond that due to improved boiling
performance alone. In other words, the enhanced nucleate boiling
surface facilitates improved heat transfer on the opposite tube
wall surface as a result of the reduction in the tube metal
temperature.
The resulting improvement in boiling and condensing heat transfer
reduces the size required for the boiling section of the heat
transfer zone and thus permits more of the available surface area
to be used in gas heating. As a result, higher outlet feed
temperatures are achieved at constant feed rates, reactor
temperatures, and exchanger geometry. Alternately, the improved
heat transfer performance from the enhanced boiling surface can be
used to operate the reforming process at a higher throughput while
maintaining the same outlet temperatures, reactor temperature, and
exchanger geometry. Therefore, by the addition of a small amount of
an enhanced nucleate boiling surface, a significant improvement in
the total heat recovery from the feed effluent exchanger can be
obtained. The improvement in heat recovery can be 10% or more. Such
a large recovery would not be feasible by merely adding additional
bare tube surface.
The use of the enhanced nucleate boiling surface can benefit the
process in many and different ways. Existing exchanger bundles in a
feed effluent exchanger can be replaced with new bundles that use
the enhanced boiling surface to thereby provide closer temperature
approaches and increase the vaporized feed outlet temperature.
Increasing the vaporized feed outlet temperature would result in
energy savings by reducing the amount of make-up fuel which must be
burned to heat the feed to the desired reactor temperature.
Increasing the vaporized feed outlet temperature can also eliminate
the need to modify or replace the old fired preheaters that are
used to raise the temperature of the reforming feed to the final
reactor temperature. Alternately, the enhanced nucleate boiling
surface in the lower portion of the heat exchanger can be used to
permit an increase in feed rate, and thus capacity, of the
exchanger for a given feed inlet and outlet temperature without
increasing the surface area or adding exchangers to the process. In
either case, the improved results are attained without additional
heat exchange surface area exchanger length which has the benefit
of not increasing the pressure drop in the exchanger section of the
process. The recovery of additional heat from the effluent stream
also has the advantage of condensing additional effluent material
thereby reducing the amount of further cooling that is needed on
the reforming effluent stream.
Accordingly, in one embodiment, this invention is a process for
reforming hydrocarbons that comprises contacting a reforming feed
with a reforming catalyst in a reforming reaction zone to form a
reforming effluent stream wherein heat from the reforming effluent
stream is transferred to the reforming feedstream by indirect heat
exchange in a vertical heat exchanger. The reforming feedstream
enters a lower end of the heat exchanger and is vaporized in a
lower section of the heat exchanger by contact with a first heat
exchange surface and the reforming effluent stream enters an upper
end of the heat exchanger and is condensed in a lower portion of
the heat exchanger. The process is improved by contacting the
reforming feedstream with an enhanced nucleate boiling surface
formed on the first heat exchange surface. The addition of the
enhanced nucleate boiling surface increases the effectiveness of
the condensing heat transfer in the lower portion of the heat
exchanger.
In another embodiment, this invention is a process for reforming
hydrocarbons that comprises combining a liquid phase reforming
feedstream and a hydrogen-rich recycle gas to form a combined
reforming feed, and passing the combined reforming feed into the
tube side and to the bottom of a vertically-oriented tube and shell
heat exchanger to heat and vaporize the feed. The vaporized feed is
passed to a reforming reaction zone and contacted therein with a
reforming catalyst to form a gas phase reforming effluent stream
that is passed to the top of the heat exchanger to transfer heat
from the reforming effluent stream to the reforming feedstream by
indirect heat exchange in the heat exchanger. At least a portion of
the reforming effluent stream is condensed in a lower portion of
the heat exchanger. This invention improves the process by
contacting the reforming feedstream with an enhanced nucleate
boiling surface formed on at least the inside and lower portion of
the tubes in the heat exchanger to increase the effectiveness of
boiling and condensing heat transfer in the lower portion of the
heat exchanger.
Other objects, embodiments and details of this invention are
described in the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified schematic flowscheme of a reforming
process.
FIG. 2 is a schematic drawing of a feed effluent heater.
FIGS. 3 and 4 are heat release curves (also referred to as heating
and cooling curves.)
DETAILED DESCRIPTION OF THE INVENTION
This invention is generally directed to improving the operation of
a reforming process. The improvement is obtained by a change to the
operation of the main heat exchanger in the reforming process which
is referred to as the feed effluent exchanger. It is desirable to
the successful practice of this invention that the feed effluent
exchanger to which this improvement is applied is a vertical type
heat exchanger. An essential feature of this invention is the use
of an enhanced nucleate boiling surface on the portion of the
exchanger in which the reforming feedstream undergoes boiling to
produce a vaporized feedstream. Another essential feature of this
invention is the partial condensation of the reactor effluent
stream on the opposite side of the heat exchange surface upon which
boiling of the feedstream is effected. In such exchangers it has
been surprisingly discovered that the overall heat transfer
coefficient of an integral vertical shell tube heat exchanger can
be improved by as much as 10-20% by the simple addition of the
enhanced nucleate boiling surface to that portion of the heat
exchanger tubes in which boiling of the feed takes place. Although
it is known that enhanced nucleate boiling surfaces can provide
significant increases in the heat transfer coefficient in a boiling
heat transfer zone, such a large increase in overall improvement
was unexpected. It is believed that the large increase is the
result of enhancing the nucleate boiling surface on a portion of
the heat exchange tube that is undergoing condensing/cooling on the
opposite side of the same portion of the heat exchange tube.
Because of the high heat transfer rates associated with the
condensing of the reforming effluent stream on the opposite
surface, the benefit from the enhanced nucleate boiling surface is
greatly and unexpectedly increased.
The operation of the reforming process can be more fully
appreciated from FIG. 1. With reference to FIG. 1, a reforming feed
enters the process through a line 10 and is combined with a recycle
gas stream carried by a line 12 that consists primarily of hydrogen
and light gases. The combined reforming feed is carried by a line
14 into the bottom of a feed effluent exchanger 16. The feed is
heated in exchanger 16 to a temperature in excess of 800.degree. F.
and withdrawn from an upper end of the exchanger through a line 18.
Line 18 carries the heated combined feed through a preheater 20
that heats the combined feed to the final reaction temperature,
usually in excess of 900.degree. F. Line 22 carries the heated
combined feed from the preheater to a reactor section 24. After
contact with a reforming catalyst, the combined feed forms a
reactor effluent stream that is carried by a line 26 from the
reaction zone to the feed effluent exchanger. The effluent is
cooled and partially condensed against the combined feedstream in
the feed effluent exchanger 16 and withdrawn from the bottom of the
exchanger by a line 28. The cooled effluent from the feed effluent
exchanger may be further condensed in a condensing unit not shown
and then transferred to a separator 30. The liquid portion
recovered in the separator, consisting of an aromatics-rich phase
and an LPG portion, is transferred by a line 32 to a stabilizer 34
and further distilled to recover a reformate product taken by line
36, an LPG stream taken by line 38, and a light off-gas stream
withdrawn through a line 40. A hydrogen-rich gaseous phase of the
cooled reforming effluent stream is recovered overhead from
separator 30 by line 42, passed through a compressor 44 to increase
its pressure, and combined with the feed by a line 12 in the manner
hereinbefore described.
The Figure represents a very simplified representation of a
reforming process. The simple representation of the reforming
process is not meant to be a limitation on the type of reforming
process configurations in which this invention may be used. For
example, it is common in reforming processes to use a series of
reactors in a reaction zone with interstage heating between
individual reactors. Likewise, a variety of arrangements for the
separation of the reforming effluent stream are known. Accordingly,
this invention may be used with any type of reaction zone and feed
or product recovery facilities.
The operation and arrangement of the feed effluent exchanger is an
important aspect of this invention. Suitable feed effluent
exchangers for use in this invention will have a vertical
orientation. FIG. 1 refers to three types of heat exchange within
zones of the heat exchanger. These three types of heat exchange
refer to the conditions on opposite sides of the heat exchange
surface and include boiling-condensing (B-C) where boiling of the
feed occurs on one side of the heat exchange surface, and partial
condensing of the effluent occurs on the other side of the heat
exchange surface, boiling-gas (B-G) where feed boiling occurs on
one side of the heat exchange surface and heat is transferred from
gas on the opposite side of the heat exchange surface, and gas-gas
(G-G) heat exchange where there is gas present on both sides of the
heat exchange surface. For the practice of this invention, the
various zones of heat exchange can be provided in a series of heat
exchangers. The exchanger must have sufficient length such that
boiling of the feedstream and condensation of the effluent stream
will occur in the same vertical section of the heat exchanger. It
is preferred that at minimum the first effluent exchanger in such a
series would have sufficient length for all of the boiling of the
feed material to occur in a single vertical section. It is further
preferred that the total heating and cooling of the reforming feed
and reforming effluent stream be performed in one or more parallel
vertical exchangers. Such exchangers are referred to as integral
exchangers. Thus, it is preferred that the exchangers are integral
and have sufficient heat exchange surface so that the reforming
feed and reforming effluent enter or aliquot portions thereof and
leave the same exchanger thereby causing boiling-condensation and
gas-gas heat transfer to occur in the same heat exchanger.
FIG. 2 shows the preferred embodiment of this invention where all
of the heat transfer zones are contained in a single integral heat
exchanger and further shows the relative location of such zones.
The (B-C) zone is at the lowest section of the heat exchanger. In
this section the temperature of the cold feed is essentially at its
boiling point and localized boiling is occurring at the heat
exchange surface that is in contact with the feedstream and the
feed comprises a mixed phase fluid of gas and liquid. On the
opposite side of the heat exchange surface, the reformer effluent
is also present as a mixed phase fluid. Condensing of the reformer
effluent occurs first on the heat exchange surface which has a
temperature below the dewpoint of the reformer effluent stream. The
division between the (B-C) zone and (B-G) zone is at the point
where the heat exchange surface in contact with the reactor
effluent stream has a temperature that is below the dewpoint of the
reformer effluent stream. Note that the upper limit of the
condensing zone is defined by the temperature of the heat exchange
surface and not the average temperature of the reformer effluent
stream therein. Consequently, in the upper end of the (B-C) zone,
the reformer effluent will have an average temperature that is
above its dewpoint, however, condensation will occur locally at the
heat exchange surface which is below the temperature of the
effluent dewpoint. At a lower point, in the (B-C) zone, the
effluent stream temperature will drop to the point where it becomes
saturated. This section is referred to as the condensing saturated
vapor zone. A substantial advantage of this invention is that it
provides a significant length (indicated by dimension A of FIG. 2)
of condensing zone located below the point where the tube surface
is below the effluent dewpoint line and above the condensing
effluent saturated vapor zone. In the prior art practice of using
bare tubes and reforming heat exchangers, there was little length
(Dimension A) if any to such condensing zone and essentially all of
the (B-C) zone contained condensing saturated vapor. With the use
of the enhanced boiling surface of this invention, the length of
the condensing zone that is free of saturated vapor will have a
length that is equal to at least 10% of the total length of the
(B-C) zone or at least 6 inches long. In many cases, the length of
the condensing zone that is free of saturated reforming feed
effluent vapor is on the order of 1 to 2 feet.
Above the (B-C) zone there is an additional zone of boiling that is
shown in FIG. 2 as zone (B-G). This is a zone where boiling of the
reforming feed occurs on one side of the heat exchange surface as a
result of gas heating from the reforming effluent vapors on the
opposite side of the heat exchange surface. This condition exists
between the points in the exchanger where the heat exchange surface
on the reforming effluent side of the heat exchanger is at the
reforming effluent dewpoint, and the heat exchange surface on the
reforming feed side of the heat exchange surface is at the
temperature of the feed dewpoint.
The use of the enhanced nucleate boiling surface of this invention
generally operates to reduce the overall combined length of zones
(B-C) and (B-G) while reducing the relative length of zone (B-G) to
zone (B-C). The reduced combined length of zones (B-C) and (B-G) is
the result of the improved heat transfer coefficient in the boiling
region that vaporizes the reforming feed more quickly. The reduced
relative length of zone (B-G) to zone (B-C) is a particular
advantage of this invention in that it provides a longer zone of
the more efficient boiling condensing heat transfer as opposed to
the less efficient boiling gas heat transfer.
Once the temperature of the heat exchange surface that is in
contact with the reforming feed has reached the feed dewpoint, the
rest of the heat transfer between the reforming feed and the
reforming effluent is gas-gas heat transfer and is shown in FIG. 2
as zone (G-G). For a fixed length of exchanger, this invention has
the advantage of providing a longer length for zone (G-G) by
reducing the required combined length of zones (B-C) and (B-G).
Where an integral heat exchanger is used, the total vertical
shell-tube length of such a unit can vary from 40 to 70 ft. This
invention can be used in any type of vertical heat exchanger.
Typically, the invention can be used in a plate-type heat exchanger
and more commonly in a shell-and-tube type heat exchanger.
The addition of the enhanced nucleate boiling surface can be
accomplished in a variety of ways. A number of patents related to
such surfaces have been enumerated in the background of this
invention. Typically, these enhanced nucleate boiling surfaces are
incorporated on the tubes of a shell-and-tube type heat exchanger.
These enhanced tubes are made in a variety of different ways which
are well known to those skilled in the art. For example, such tubes
may comprise annular or spiral cavities extending along the tube
surface made by mechanical working of the tube. Alternately, fins
may be provided on the surface. In addition the tubes may be scored
to provide ribs, grooves, a porous layer and the like.
Generally, the more efficient enhanced tubes are those having a
porous layer on the boiling side of the tube. The porous layer can
be provided in a number of different ways well known to those
skilled in the art. The most efficient of these porous surfaces
have what are termed reentrant cavities that trap vapors in
cavities of the layer through restricted cavity openings. In one
such method, as described in U.S. Pat. No. 4,064,914, the porous
boiling layer is bonded to one side of a thermically conductive
wall. The porous boiling layer is made of thermally conductive
particles bonded together to form interconnected pores of capillary
size having equivalent pore radius of less than about 6.0 mils, and
preferably less than about 4.5 mils. As used herein, the phrase
"equivalent pore radius" empirically defines a porous boiling
surface layer having varied pore sizes and non-uniform pore
configurations in terms of an average uniform pore dimension. Such
an enhanced tube containing a porous boiling layer is commercially
available under the tradename High Flux Tubing made by UOP, Des
Plaines, Ill.
An essential characteristic of the porous surface layer is the
interconnected pores of capillary size, some of which communicate
with the outer surface. Liquid to be boiled enters the subsurface
cavities through the outer pores and subsurface interconnecting
pores, and is heated by the metal forming the walls of the
cavities. At least part of the liquid is vaporized within the
cavity and resulting bubbles grow against the cavity walls. A part
thereof eventually emerges from the cavity through the outer pores
and then rises through the liquid film over the porous layer for
disengagement into the gas space over the liquid film. Additional
liquid flows into the cavity from the interconnecting pores and the
mechanism is continuously repeated.
When using an enhanced boiling surface other than a porous layer,
the boiling film heat transfer coefficient is typically increased
by a factor of about 4 or more to a value of at least about 400
(Btu/hr/ft.sup.2 .degree. F. By utilizing this enhanced boiling
surface, containing a porous boiling layer, the boiling film heat
transfer coefficient of the boiling fluid within the tubes can be
increased by a factor of about 10, typically to a value of 700
(Btu/hr/ft.sup.2 .degree. F.) or more. This is due to the fact that
the heat leaving the base metal surface of the tube does not have
to travel through an appreciable liquid layer before meeting a
vapor-liquid surface producing evaporation. Within the porous
layer, a multitude of bubbles are grown so that the heat, in order
to reach a vapor liquid boundary, need travel only through an
extremely thin liquid layer having a thickness considerably less
than the minute diameter of the confining pore. Vaporization of the
liquid takes place entirely within the pores.
The surprising benefit of adding the enhanced nucleate boiling
surface in the boiling zone of the feed effluent heat exchanger can
be more fully appreciated by the heat release curve shown in FIG.
3. FIG. 3 provides a comparison of the enhanced and bare tubes at
fixed exchanger geometry feed flow rate and reactor temperature.
The higher feed outlet temperature that is obtained by this
invention results in a reduction in the overall temperature
difference for heat transfer. FIG. 3 illustrates the mechanism for
this reduction. The curve, A, B, C, represents the heat released to
the feed in an exchanger that does not use the enhanced nucleate
boiling surface of this invention. Feed enters at point A and
vaporizes to the dewpoint B. With the use of bare tubes, gas leaves
the exchanger at point C. Again, for the case of a bare tube heat
exchanger, curve G, F, E, represents the heat released from the
reformer effluent stream. The effluent enters the exchanger at
point E and begins to release heat to the feedstream. Eventually,
it reaches the effluent dewpoint, point F. Partial condensing then
occurs below point F until the point G where the reformer effluent
is removed from the exchanger. Additional partial condensing may
also occur along the region defined by points E and F where the
tube metal temperature is below the dewpoint of the reactor
effluent stream.
When an enhanced boiling surface is used, there is additional
length in the gas-gas heat exchange zone as represented by that
portion of the heat release curve between point C and D.
Accordingly, the extra heat recovered by the feedstream in this
invention is represented by segment C, D. Since the effluent always
has the same inlet temperature, and the mass flow rate for the
reformer effluent is the same as the feed, a new heat release curve
for the effluent stream is shown by the dotted line in FIG. 3. The
heat release curve that is representative of this invention begins
above the bare tube heat release curve G, F, E at a point E'. It is
apparent that the overall temperature difference between the feed
and effluent has decreased relative to the bare tube case and that
the condensing zone G' to F' has increased in heat load relative to
the condensing zone for the bare tube case represented by segment
G, F. The upper displacement of heat release curve G', F', and E',
demonstrates the major reason why conventional feed effluent
heaters are difficult to improve in terms of heat recovery since an
increment in heat recovery is matched by a reduction in available
temperature difference. A key feature of this invention is that it
overcomes some of this handicap by promoting improved heat transfer
coefficients in the condensing zone which tend to compensate for
the reduced temperature difference across the heat exchanger and
conditions approaching a "pinch", i.e., where curve G, F, E moves
upward until it approaches line A, B, C to a point where no further
heat recovery is possible.
As FIG. 3 demonstrates, the method of this invention offers
significant heat recovery advantages. As briefly mentioned in the
summary of the invention, this improved heat removal capacity can
be used to either obtain an improved end point temperature for the
reforming feedstream as it leaves the heat exchanger or to increase
the capacity of an existing exchanger. There are several
possibilities when the invention is used to increase the throughput
or capacity of the exchanger. In such alternatives, existing system
pressure drop is a major consideration. For systems that currently
have a low pressure drop, the use of enhanced tubes can increase
the overall heat transfer coefficient about 40%. Therefore, at
fixed stream temperature differences the capacity can be increased
by 40% with equal size exchangers and with only a doubling of the
pressure drop.
The improved feed effluent heat exchangers that use the enhanced
nucleate boiling service of this invention can be used in both new
or existing plant environments. The most useful application would
be in the retrofit or replacement of existing tube bundles for
shell and tube heat exchangers to improve the operation of process
conditions.
In addition to adding the enhanced nucleate boiling surface, other
enhancement of the heat exchange surfaces in different heat
transfer zones of the exchanger may also be used. For example,
other surfaces can be added in the condensing zone to further
promote condensing of liquid on the tube surface. Known methods for
enhancing a condensing surface include the use of a "sand grain"
surface that has large heat conductive particles on the heat
exchange surface to provide local cold spots that will initiate
condensation. Other enhancements that can be used would be in the
gas heat transfer zones. Some examples of known enhancements
include twisted tape inserts or low fin tubing, each of which
increase the heat exchange surface for the transfer of sensible
heat in the gas zones. Such additional enhancement surfaces would
increase the overall heat transfer capability of the exchanger and
further increase heat recovery without changing any of the benefits
of the enhanced nucleate boiling surface as discussed above.
Whether the enhanced nucleate boiling surface is used in old or new
heat exchangers, there are few restrictions on its application. The
only essential element of the invention is that the enhanced
nucleate boiling surface be provided in the boiling zone of the
heat exchanger. The boiling zone in the corresponding enhanced
nucleate boiling surface can be used on most types of heat exchange
surfaces. In the case of a shell-and-tube type heat exchanger, the
enhanced nucleate boiling surface may be placed on either the
inside or outside of the tubes. However, it is generally preferred
to place the enhanced nucleate boiling surface and have boiling
occur on the inside of the tubes in a vertical shell-and-tube type
heat exchanger. Also, the enhanced nucleate boiling surface should
extend throughout the entire boiling zone to obtain maximum benefit
from the invention. This means that in a typical heat exchanger
tube, approximately the first 40% of the tube will be coated with
the enhanced nucleate boiling surface. Most enhanced nucleate
boiling surfaces will not interfere with heat transfer in the gas
heat transfer zone above the boiling zone. Therefore, the enhanced
nucleate boiling surface should be extended upward through to what
is considered the maximum distance that the boiling zone can
extend. In this regard the entire reformer feed side of the heat
exchanger may be provided with the enhanced nucleate boiling
surface. It does appear that there may be some small heat transfer
advantages to providing the enhanced nucleate boiling surface over
the entire length of the heat exchanger on the reforming feed side
of the heat transfer surface, if cost considerations are
disregarded.
EXAMPLE
The following example illustrates the potential benefits gained by
application of the invention in replacement of the feed-effluent
exchangers in a 35,000 bpd catalytic reformer with tubes containing
a High Flux boiling surface in the lower boiling zone portion. This
example is based in part on computer simulations and engineering
calculations that are derived from operating units. The existing
system which is representative of the prior art operates with 2
shell-tube feed-effluent exchangers in parallel, and a flow scheme
similar to FIG. 1. Each exchanger in the existing system has 3,000
bare tubes that are 40 ft. in length and 3/4 in. in diameter. The
nucleate boiling surface of this invention was provided by adding 2
new exchangers each containing 3000, 3/4 in. diameter tubes. All
the tubes are 40 ft. long and the High Flux boiling surface is
applied to the bottom 16 ft. of the tube interior.
The High Flux surface substantially increases the overall heat
transfer coefficient in the boiling zone. The High Flux exchangers
were calculated to have an overall U-value for the entire exchanger
of about 60 Btu/hr/ft.sup.2 /F. The existing exchangers in
operation had overall measured heat transfer coefficients of about
35 Btu/hr/ft.sup.2 /F. and expected or calculated values of about
50, which is typical of vertical exchanger performance in various
units of this type.
Heat transfer coefficients for the bare tube exchangers have been
calculated using well-known procedures familiar to heat exchanger
designers. The large difference between the calculated and measured
heat transfer values for the existing exchangers is due to
differences between the theoretical and actual environment in the
heat exchanger which has a major impact on the convective heat
transfer coefficients for the existing tubes. The overall heat
transfer coefficient for the existing exchangers is controlled in
large part by these convective heat transfer coefficients. The
calculated overall heat transfer coefficient for the tubes that use
the High Flux surface is more reliable since the High Flux
coefficient is not dependent on convective coefficients and,
therefore, not greatly influenced by changes in flow regime High
Flux exchangers were simulated with procedures modified to correct
for the heat transfer effects herein described.
A summary of the operating conditions for the existing exchangers
and High Flux exchangers is shown in Table 1. The naphtha and
recycle gas feed enter the tube side at the bottom with the liquid
portion comprising about 84% by weight of the combined feed, and at
a temperature of 180.degree. F. Four cases were considered in this
example. The simulated performance is shown in column 1 for the
case of High Flux tubing and the 35,000 bpd feed rate. Simulated
and observed performance for the bare tube exchanger case at the
35,000 bpd feed rate appears in columns 2 and 3. Column 4
illustrates a case for increased capacity where the feed rate is
raised from 35,000 to 50,000 bpd, while maintaining terminal
temperatures similar to column 2.
Referring again to Table 1, the overall heat transfer coefficients
for the High Flux case are 44 and 90, respectively, in the gas-gas
and boiling zones, and the overall total value is 58. The bare tube
values, based on observed performance are 33 and 39 in the gas and
boiling zones and 35 overall. The contribution of the High Flux
surface, depending on whether case 2 or 3 is considered, increases
the boiling zone coefficient from 1.6 to 2.6 fold beyond the bare
tube value.
FIG. 4 shows the heat release curves for the High Flux example, and
for comparison, the effluent cooling curve expected when the
replaced bare tube exchangers exhibited the observed performance.
The region between the 2 effluent curves represents the additional
recovered heat with the enhanced boiling surface retrofit.
The total heat transferred is also shown in Table 1. After the
exchanger replacement, about 245 million Btu/hr are recovered,
while values of 227-240 are recovered in the bare tube cases. The
difference between these values represents the fuel savings. It is
apparent that the use of a High Flux surface on only the lower part
of the tubes saved at least 5 and most probably 18 million Btu/hr.
If preheat fuel is worth $4/MMBtu, then the annual fuel saving
ranges from $170,000 to $600,000.
TABLE 1
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Case 1 Case 2 Case 3 Case 4 High Flux Bare Tube Bare Tube High Flux
Revamp Observed Expected Capacity 35,000 B/D Operation Operation
Increase
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Total Flowrate lb/hr 427,000 427,000 427,000 605,700 Percent Liquid
Feed Inlet 84 84 84 84 Percent Liquid Effl. Out 39 25 35 36
Reformer Capacity B/D 35,000 35,000 35,000 50,000 Inlet Feed Temp.
.degree.F. 180 180 180 180 Outlet Feed Temp. .degree.F. 848 800 834
834 Inlet Effluent Temp. .degree.F. 930 930 930 930 Outlet Effluent
Temp. .degree.F. 260 293 270 270 Feed Dewpoint .degree.F. 371 371
371 371 Effluent Dewpoint .degree.F. 346 346 346 346 H.T Coeff.
Btu/hr/ft.sup.2 /F Boiling Film 750 135 130 900 Gas Zone Overall 44
33 44 55 Boiling Zone Overall 90 39 55 110 Overall, Exchanger 58 35
50 70 Surface Area ft.sup.2 Total 46,200 46,200 46,200 46,200 Heat
Transferred MMBtu/hr 245.4 227.9 240.0 340.4 Boiling Zone Length
ft. 12 16 16 12.4 Condensing Zone Length ft. 8.3 6+ 8+ 8.7
Saturated Vapor Condensing 6.3 6 8 6.0 Zone Length ft.
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